Learn what an aircraft ADIRU does, the air-data and inertial outputs it supplies, why alignment matters, and what common failures mean.
In real-world aviation, an ADIRU (Air Data Inertial Reference Unit) is an avionics unit that supplies airspeed, altitude, attitude, heading and inertial-navigation data. It combines air-data calculations from pressure and temperature inputs with gyroscopes and accelerometers, then feeds the flight displays, flight computers, autopilot and navigation system.
What data does an ADIRU provide?
An ADIRU supplies two related families of information: air data and inertial reference data. These functions may be independently selectable even though they occupy the same physical unit.
| ADIRU section | Source information | Typical outputs |
|---|---|---|
| Air Data Reference (ADR) | Pitot pressure, static pressure, temperature and, where applicable, angle-of-attack inputs | Indicated airspeed, Mach number, barometric altitude, vertical speed and air temperature |
| Inertial Reference (IR) | Gyroscopes and accelerometers sensing rotation, gravity and linear movement | Pitch, roll, heading, track, position, groundspeed and acceleration |
The pressure probes and other sensors are normally installed elsewhere on the airframe; the ADIRU receives their measurements and calculates usable flight parameters. Its inertial section establishes the aircraft's orientation and movement without relying continuously on signals from outside the aircraft. Our explanation of how aircraft attitude is defined covers the pitch, roll and yaw terminology behind these outputs.
An ADIRU is not simply a GPS receiver. Modern navigation computers combine its inertial solution with GNSS and radio-navigation updates, correcting the gradual position drift inherent in inertial navigation. We explain that integration in our guide to how aircraft navigation systems combine their data sources.
How does the aircraft use ADIRU data?
The aircraft uses ADIRU data anywhere a computer or display needs to know how high, fast, level or accurately positioned the aircraft is.
- Primary flight and navigation displays use it for speed, altitude, attitude, heading and track indications.
- Autopilot and flight-director systems use it to maintain attitude, altitude, heading and programmed flight paths.
- Flight-management computers use inertial position, groundspeed and track when calculating navigation guidance.
- Flight-control and warning computers may use airspeed, Mach, angle of attack and acceleration for control laws, protections and alerts.
- Weather radar and other equipment can use attitude data for stabilisation.
Transport aircraft normally carry two or three independent inertial or air-data sources. Their computers can compare outputs, detect disagreements and use an alternate source after a failure. The exact voting and switching logic depends on the aircraft type, and separate standby instruments provide another independent reference.
Why does an ADIRU need to align?
An ADIRU must align before normal inertial navigation because it needs to establish local vertical, true north and an accurate starting position.
During a normal ground alignment, its gyroscopes sense the Earth's rotation while its accelerometers establish the gravity reference. The aircraft must usually remain stationary, and the crew confirms or enters its initial position through the flight-management system. Alignment takes several minutes on many aircraft and can take longer, or have limitations, at high latitudes.
Moving the aircraft during alignment, entering the wrong gate position or removing electrical power can delay or invalidate the process. Some types offer a rapid realignment or an attitude-only mode, but neither should be treated as a universal substitute for a full alignment; the available functions and procedures are aircraft-specific.
Why will an ADIRU not align in a flight simulator?
A simulated ADIRU usually remains unavailable because it lacks power, has not received an initial position or was disturbed while aligning.
- Check electrical power. Confirm that the required aircraft buses and avionics are powered, not merely the cockpit displays.
- Select the normal navigation mode. On aircraft with separate IR selectors and ADR controls, make sure the appropriate channels are enabled.
- Keep the aircraft stationary. Do not release the parking brake, slew or reposition the aircraft during alignment.
- Enter or confirm the present position. Use the aircraft's flight-management interface where the simulation models this step.
- Wait for completion. Repeatedly cycling the selectors restarts the process rather than speeding it up.
Implementation depth varies between simulator aircraft. The FlyByWire A32NX package for MSFS provides a detailed example in which alignment state and inertial-system faults affect the cockpit indications.
Is an ADIRU the same as an IRS, AHRS or air-data computer?
An ADIRU is related to these systems but combines more functions in one replaceable avionics unit.
- ADC or ADR: calculates airspeed, altitude, Mach and related air-data values from pressure and temperature inputs.
- IRS or IR: supplies attitude, heading, movement and inertial position information.
- AHRS: supplies attitude and heading, usually without the complete long-range inertial-navigation capability associated with an IRS.
- ADIRU: combines air-data and inertial-reference functions in one unit.
Manufacturers do not all use these labels identically. Cockpit controls, backup units and the division of calculations between sensors and computers therefore differ between Airbus, Boeing and other aircraft families.
What happens if an ADIRU fails?
An ADIRU failure can remove only air-data information, only inertial information, or both, depending on which section or external input has failed.
- An ADR fault can produce invalid or missing airspeed, altitude or Mach indications. Blocked or faulty pitot-static sensors can create similar symptoms without the ADIRU itself being defective.
- An IR fault can remove attitude, heading, track or inertial-position data and may cause autopilot or flight-control degradation.
- A source disagreement means independent units do not match. Crews cross-check the remaining sources and standby instruments before isolating the suspect channel.
- A total unit or power failure removes both sets of outputs from that ADIRU, leaving the aircraft to use its remaining sources.
Loss of GPS is not automatically an ADIRU failure. The inertial system can continue calculating position, although accuracy gradually degrades without external updating; our explanation of why an A320 can show GPS PRIMARY LOST describes that distinction.
On a real aircraft, crews follow the type-specific checklist to identify and select valid sources. An ADIRU should not be reset or switched off in flight merely to clear a fault message unless the approved procedure explicitly directs it.